1. Field of the Invention
This invention relates to the field of transfer cases for motor vehicles; more particularly it pertains to control strategies and systems for such transfer cases.
2. Description of the Prior Art
A transfer case is a device located in a motor vehicle drive line between the output of a geared power transmission and front and rear driveshafts for transmitting power to the wheels. A transfer case may include a planetary gear set that produces both a high range, in which the transfer case output is driven at the same speed as the input, and a low range, in which the output is driven slower than its input speed. The 4×2 and 4×4 states of the transfer case are usually selected manually by the vehicle operator by operating a lever or switch. A first position of the lever causes a range selection device in the transfer case to direct power from the transmission output to a rear drive axle, the 4×2-drive mode. A second position of the lever causes the transfer case to direct power to both a front drive axle and a rear drive axle, the 4×4-drive mode.
Certain transfer case control systems either fully engage or fully disengage the secondary driveshaft and the power source. A transfer case control system for all wheel drive operation transmits power continually and variably to the front and rear driveshafts. Various techniques are available for establishing the torque split or portion of the engine torque that is transmitted to the front and rear wheels. For example, a center differential mechanism continually divides torque at a fixed ratio between the front and rear wheels, perhaps 35% of torque to the front wheels and 65% to the rear wheels. But a center differential mechanism provides no variation of the torque division as needed to improve vehicle handling under certain drive conditions. A center differential usually includes a planetary gearset having an input, such as a sun gear driven by the transmission output, a first output such as a carrier connected to the rear driveshaft, and a second output such as ring gear connected to the front driveshaft.
A viscous coupling, located in parallel with the front and rear driveshafts, or the first and second outputs of a center differential, operates to mutually connect or couple the driveshafts in proportion to the speed difference between them. It produces this effect by shearing a viscous fluid located between closely spaced plates, one set of plates driven by the front driveshaft and a second set of plates driven by the rear driveshaft. Variations in the speed difference of the sets of plates increase the magnitude of the forces tending to maintain the plates at the same speed. The coupling dissipates a portion of the output power in the process of synchronizing the speeds of the front and rear driveshafts.
A hydraulically actuated clutch continually driveably connected to a primary driveshaft can be used to transmit a variable magnitude of torque to a secondary driveshaft. The magnitude of torque transmitted to the secondary shaft is controlled electronically to improve vehicle handling characteristics under certain drive conditions. However, a transfer case clutch of this type provides a torque split to input improves vehicle handling by operating partially engaged during lengthy service periods. This partial engagement or clutch slip causes the clutch to develop heat as it attempts continually to synchronize the speed of the clutch output and the input.
A portion of the power transmitted to the clutch is dissipated there by frictional contact between the friction discs and spacer plates of the clutch. The magnitude of power dissipated varies with clutch slip, clutch speed, and the magnitude of the friction force tending to engage the discs and spates, and the length of the period of clutch slip. Friction wears the surfaces of the plates and discs and affects performance of the clutch over time, particularly if the temperature of the clutch components and fluid becomes excessive.
There is a need, therefore, for a method, system and apparatus for controlling a transfer case clutch in a four-wheel drive vehicle such that the handling characteristics of the vehicle are improved by operating the clutch partially engaged, yet excess heat accumulation in the clutch is avoided predictably and reliably.